Deposition of manganese onto ferritic and other surfaces has heretofore met with little success. The present invention teaches a method of depositing manganese and other hard-to-deposit metals such as titanium, vanadium, and zirconium.
Alloys of steel containing manganese (Mn) and about 1 to 1.2% carbon exhibit excellent hardness, average ductility, and excellent resistance to corrosion and wear. This is due to the presence of a highly stable MnO.sub.2 film that forms on the surface. This oxide film is superior to nickel and chromium oxide films which do not effectively protect steel against wear and corrosion. Ferro-manganese steel has historically been prepared by casting or thermal processes. These processes are primitive, time-consuming, expensive, and generally produced steels high in sulfur and phosphorous.
Prior art processes for deposition of manganese have been taught by Campbell, achieving a deposition of 10% manganese. The present invention has demonstrated a deposition of manganese at about 38%. Campbell reported on the deposition of manganese nickel and manganese iron alloys. (See A. L. Campbell, Electrolytic formation of alloys and amalgams of manganese, F. Chem. Society, 125, 1713-1719 (1924). Campbell was trying to determine whether codeposition of iron-group metals with manganese resulted in anomalous deposition potentials as was observed in the deposition of the iron-group metals with zinc. The bath of Campbell contained the sulfates of manganese, the iron-group metal and ammonia. Campbell obtained bright, smooth deposits at a current density of 8 MA/cm.sup.2 containing about 10% manganese. The cathode current density was about 15%. The baths of Campbell were also used by Agladze and Gdzelishvili who investigated the anodic dissolution of iron-manganese in an electrolyte solution containing 83% manganese and 8% iron and ammonium sulfates to deposit a manganese iron alloy onto the cathode. (See, R. I. A. Gladze and M. Y. A. Gdzelishvili, Electrodeposition of Iron-Manganese Alloys, Soobsh. Akad. Nauk Gruzinskoi S. R. 9, 555-562 (1949). At current density less than about 3 am/dm.sup.2, 15.degree. C.; pH 3.1, the anode dissolved to yield manganese and ferric ions. Some iron hydroxide precipitated and caused passivation of the anode. Gladze showed that at higher current densities manganese went into solution as ammonium permanganate, and iron entered the solution as ferric ions. The cathode contained between 2 and 16% iron, and the composition of the deposit was not significantly affected by variations in plating conditions.
Manganese alloys are difficult to deposit because manganese is very electronegative in aqueous solutions. As a general rule, co-deposition takes place when the electronegativities of its components parts are within 200 mV of each other. Manganese, having an electronegativity of
1.18 volts, is far removed from the potentials of other depositable metals. See, Brenner, Abner; Electrodeposition of Alloys: Principle and Practice, Vols. 1 and 11, Academic Press, New York and London, 1963. One approach to effectively deposit manganese alloys is the use of complexing agents to shift the deposition potential of a more noble metal to closer to that of manganese to bring about codeposition. This approach has been largely unsuccessful because the complexing agent degrades the quality of the deposit. Manganese alloys (except copper, tin and nickel) are deposited from a slightly acid bath of simple ions. In addition to the difficulty of deposition, manganese has three drawbacks including brittleness, dark black color and high reactivity. The chief use of electrodeposited manganese or its alloys is protective coatings for steel or other metals. Brenner concluded that the deposition of manganese alloys of the iron group metals has four characteristics: namely, (a) the alloys contained low percentages of manganese usually not exceeding 12%; (b) the alloys deposited at low cathode current efficiencies of about 20%; (c)iron manganese could only be deposited in a mildly acid salt solution; and (d) at higher pH, hydroxide precipitated and the bath became inoperable.